Voltage-gated sodium channels, transmembrane proteins that initiate action potentials in nerve, muscle and other electrically excitable cells, are a necessary component of normal sensation, emotions, thoughts and movements (Catterall, W. A., Nature (2001), Vol. 409, pp. 988-990). These channels consist of a highly processed alpha subunit that is associated with auxiliary beta subunits. The pore-forming alpha subunit is sufficient for channel function, but the kinetics and voltage dependence of channel gating are in part modified by the beta subunits (Goldin et al., Neuron (2000), Vol. 28, pp. 365-368). Each alpha-subunit contains four homologous domains, I to IV, each with six predicted transmembrane segments. The alpha-subunit of the sodium channel, forming the ion-conducting pore and containing the voltage sensors regulating sodium ion conduction has a relative molecular mass of 260,000. Electrophysiological recording, biochemical purification, and molecular cloning have identified ten different sodium channel alpha subunits and four beta subunits (Yu, F. H., et al., Sci. STKE (2004), 253; and Yu, F. H., et al., Neurosci. (2003), 20:7577-85).
The hallmarks of sodium channels include rapid activation and inactivation when the voltage across the plasma membrane of an excitable cell is depolarized (voltage-dependent gating), and efficient and selective conduction of sodium ions through conducting pores intrinsic to the structure of the protein (Sato, C., et al., Nature (2001), 409:1047-1051). At negative or hyperpolarized membrane potentials, sodium channels are closed. Following membrane depolarization, sodium channels open rapidly and then inactivate. Channels only conduct currents in the open state and, once inactivated, have to return to the resting state, favoured by membrane hyperpolarization, before they can reopen. Different sodium channel subtypes vary in the voltage range over which they activate and inactivate as well as their activation and inactivation kinetics.
The sodium channel family of proteins has been extensively studied and shown to be involved in a number of vital body functions. Research in this area has identified variants of the alpha subunits that result in major changes in channel function and activities, which can ultimately lead to major pathophysiological conditions. Implicit with function, this family of proteins are considered prime points of therapeutic intervention. Nav1.1 and Nav1.2 are highly expressed in the brain (Raymond, C. K., et al., J. Biol. Chem. (2004), 279(44):46234-41) and are vital to normal brain function. In humans, mutations in Nav1.1 and Nav1.2 result in severe epileptic states and in some cases mental decline (Rhodes, T. H., et al., Proc. Natl. Acad. Sci. USA (2004),101(30):11147-52; Kamiya, K., et al., J. Biol. Chem. (2004), 24(11):2690-8; Pereira, S., et al., Neurology (2004), 63(1):191-2). As such both channels have been considered as validated targets for the treatment of epilepsy (see PCT Published Patent Publication No. WO 01/38564).
Nav1.3 is broadly expressed throughout the body (Raymond, C. K., et al., op. cit.). It has been demonstrated to have its expression upregulated in the dorsal horn sensory neurons of rats after nervous system injury (Hains, B. D., et al., J. Neurosci. (2003), 23(26):8881-92). Many experts in the field have considered Nav1.3 as a suitable target for pain therapeutics (Lai, J., et al., Curr. Opin. Neurobiol. (2003), (3):291-72003; Wood, J. N., et al., J. Neurobiol. (2004), 61(1):55-71; Chung, J. M., et al., Novartis Found Symp. (2004), 261:19-27; discussion 27-31, 47-54).
Nav1.4 expression is essentially limited to muscle (Raymond, C. K., et al., op. cit.). Mutations in this gene have been shown to have profound effects on muscle function including paralysis, (Tamaoka A., Intern. Med. (2003), (9):769-70). Thus, this channel can be considered a target for the treatment of abnormal muscle contractility, spasm or paralysis.
The cardiac sodium channel, Nav1.5, is expressed mainly in the heart ventricles and atria (Raymond, C. K., et al., op. cit.), and can be found in the sinovial node, ventricular node and possibly Purkinje cells. The rapid upstroke of the cardiac action potential and the rapid impulse conduction through cardiac tissue is due to the opening of Nav1.5. As such, Nav1.5 is central to the genesis of cardiac arrhythmias. Mutations in human Nab1.5 result in multiple arrhythmic syndromes, including, for example, long QT3 (LQT3), Brugada syndrome (BS), an inherited cardiac conduction defect, sudden unexpected nocturnal death syndrome (SUNDS) and sudden infant death syndrome (SIDS) (Liu, H. et al., Am. J. Pharmacogenomics (2003), 3(3):173-9). Sodium channel blocker therapy has been used extensively in treating cardiac arrhythmias. The first antiarrhythmic drug, quinidine, discovered in 1914, is classified as a sodium channel blocker.
Nav1.6 encodes an abundant, widely distributed voltage-gated sodium channel found throughout the central and peripheral nervous systems, clustered in the nodes of Ranvier of neural axons (Caldwell, J. H., et al., Proc. Natl. Acad. Sci. USA (2000), 97(10): 5616-20). Although no mutations in humans have been detected, Nav1.6 is thought to play a role in the manifestation of the symptoms associated with multiple sclerosis and has been considered as a target for the treatment of this disease (Craner, M. J., et al., Proc. Natl. Acad. Sci. USA (2004), 101(21):8168-73).
Nav1.7 was first cloned from the pheochromocytoma PC12 cell line (Toledo-Aral, J. J., et al., Proc. Natl. Acad. Sci. USA (1997), 94:1527-1532). Its presence at high levels in the growth cones of small-diameter neurons suggested that it could play a role in the transmission of nociceptive information. Although this has been challenged by experts in the field as Nav1.7 is also expressed in neuroendocrine cells associated with the autonomic system (Klugbauer, N., et al., EMBO J. (1995), 14(6):1084-90) and as such has been implicated in autonomic processes. The implicit role in autonomic functions was demonstrated with the generation of Nav1.7 null mutants; deleting Nav1.7 in all sensory and sympathetic neurons resulted in a lethal perinatal phenotype. (Nassar, et al., Proc. Natl. Acad. Sci. USA (2004), 101(34):12706-11.). In contrast, by deleting the Nav1.7 expression in a subset of sensory neurons that are predominantly nociceptive, a role in pain mechanisms, was demonstrated (Nassar, et al., op. cit.). Further support for Nav1.7 blockers active in a subset of neurons is supported by the finding that two human heritable pain conditions, primary erythermalgia and familial rectal pain, have been shown to map to Nav1.7 (Yang, Y., et al., J. Med. Genet. (2004), 41(3):171-4).
The expression of Nav1.8 is essentially restricted to the DRG (Raymond, C. K., et al., op. cit.). There are no identified human mutations for Nav1.8. However, Nav1.8-null mutant mice were viable, fertile and normal in appearance. A pronounced analgesia to noxious mechanical stimuli, small deficits in noxious thermoreception and delayed development of inflammatory hyperalgesia suggested to the researchers that Nav1.8 plays a major role in pain signalling (Akopian, A. N., et al., Nat. Neurosci. (1999), 2(6): 541-8). Blocking of this channel is widely accepted as a potential treatment for pain (Lai, J, et al., op. cit.; Wood, J. N., et al., op. cit.; Chung, J. M., et al., op. cit.). PCT Published Patent Application No. WO03/037274A2 describes pyrazole-amides and sulfonamides for the treatment of central or peripheral nervous system conditions, particularly pain and chronic pain by blocking sodium channels associated with the onset or recurrance of the indicated conditions. PCT Published Patent Application No. WO03/037890A2 describes piperidines for the treatment of central or peripheral nervous system conditions, particularly pain and chronic pain by blocking sodium channels associated with the onset or recurrence of the indicated conditions. The compounds, compositions and methods of these inventions are of particular use for treating neuropathic or inflammatory pain by the inhibition of ion flux through a channel that includes a PN3 (Nav1.8) subunit.
The tetrodotoxin insensitive, peripheral sodium channel Nav1.9, disclosed by Dib-Hajj, S. D., et al. (see Dib-Hajj, S. D., et al., Proc. Natl. Acad. Sci. USA (1998), 95(15):8963-8) was shown to reside solely in the dorsal root ganglia. It has been demonstrated that Nav1.9 underlies neurotrophin (BDNF)-evoked depolarization and excitation, and is the only member of the voltage gated sodium channel superfamily to be shown to be ligand mediated (Blum, R., Kafitz, K. W., Konnerth, A., Nature (2002), 419 (6908):687-93). The limited pattern of expression of this channel has made it a candidate target for the treatment of pain (Lai, J, et al., op. cit.; Wood, J. N., et al., op. cit.; Chung, J. M. et al., op. cit.).
NaX is a putative sodium channel, which has not been shown to be voltage gated. In addition to expression in the lung, heart, dorsal root ganglia, and Schwann cells of the peripheral nervous system, NaX is found in neurons and ependymal cells in restricted areas of the CNS, particularly in the circumventricular organs, which are involved in body-fluid homeostasis (Watanabe, E., et al., J. Neurosci. (2000), 20(20):7743-51). NaX-null mice showed abnormal intakes of hypertonic saline under both water- and salt-depleted conditions. These findings suggest that the NaX plays an important role in the central sensing of body-fluid sodium level and regulation of salt intake behaviour. Its pattern of expression and function suggest it as a target for the treatment of cystic fibrosis and other related salt regulating maladies.
Studies with the sodium channel blocker tetrodotoxin (TTX) used to lower neuron activity in certain regions of the brain, indicate its potential use in the treatment of addiction. Drug-paired stimuli elicit drug craving and relapse in addicts and drug-seeking behavior in rats. The functional integrity of the basolateral amygdala (BLA) is necessary for reinstatement of cocaine-seeking behaviour elicited by cocaine-conditioned stimuli, but not by cocaine itself. BLA plays a similar role in reinstatement of heroin-seeking behavior. TTX-induced inactivation of the BLA on conditioned and heroin-primed reinstatement of extinguished heroin-seeking behaviour in a rat model (Fuchs, R. A. and See, R. E., Psychopharmacology (2002) 160(4):425-33).
This closely related family of proteins has long been recognised as targets for therapeutic intervention. Sodium channels are targeted by a diverse array of pharmacological agents. These include neurotoxins, antiarrhythmics, anticonvulsants and local anesthetics (Clare, J. J., et al., Drug Discovery Today (2000) 5:506-520). All of the current pharmacological agents that act on sodium channels have receptor sites on the alpha subunits. At least six distinct receptor sites for neurotoxins and one receptor site for local anesthetics and related drugs have been identified (Cestéle, S. et al., Biochimie (2000), Vol. 82, pp. 883-892).
The small molecule sodium channel blockers or the local anesthetics and related antiepileptic and antiarrhythmic drugs, interact with overlapping receptor sites located in the inner cavity of the pore of the sodium channel (Catterall, W. A., Neuron (2000), 26:13-25). Amino acid residues in the S6 segments from at least three of the four domains contribute to this complex drug receptor site, with the IVS6 segment playing the dominant role. These regions are highly conserved and as such most sodium channel blockers known to date interact with similar potency with all channel subtypes. Nevertheless, it has been possible to produce sodium channel blockers with therapeutic selectivity and a sufficient therapeutic window for the treatment of epilepsy (e.g. lamotrignine, phenyloin and carbamazepine) and certain cardiac arrhythmias (e.g. lignocaine, tocamide and mexiletine). However, the potency and therapeutic index of these blockers is not optimal and have limited the usefulness of these compounds in a variety of therapeutic areas where a sodium channel blocker would be ideally suited.